1. Field of the Invention
The present invention relates to a pixel clock generating device, a laser scanning device, and an image forming device.
2. Description of the Related Art
FIG. 25 shows a general configuration of an image forming device such as a laser printer, or a digital copy machine. As shown in FIG. 25, a laser beam emitted from a semiconductor laser unit 1009 is scanned by a rotational polygon mirror 1003, and is reflected to a photo conductor 1001 that functions as an object medium. The light spot of the laser beam on the photo conductor 1001 forms an electrostatic latent image there. Once the polygon mirror 1003 finishes the scanning of one line, it deflects the laser beam to irradiate a photo detector 1004. The photo detector 1004 converts the irradiated laser beam into electrical signals and inputs the signals to a PLL (Phase Locked Loop) circuit 1006. The PLL circuit generates a clock signal (called “pixel clock” or “image clock”) for the next line after receiving the signals from the photo detector 1004. Further, a clock signal having a frequency higher than the pixel clock (hereinafter, referred to as “high frequency clock”) is input to the PLL circuit 1006 from a clock generator 1005, thereby the phase of the pixel clock signal is synchronized.
The pixel clock signal generated in the PLL circuit 1006 is supplied to an image processing unit 1007 and a laser driving circuit 1008. The image processing unit 1007 inputs image data to the laser driving circuit 1008 according to the pixel clock, and the laser driving circuit 1008 drives the semiconductor laser unit 1009 on the basis of the pixel clock. Consequently, an electrostatic latent image corresponding to the input image data is formed on the photo conductor 1001.
As shown above, according to the pixel clock whose phase is adjusted in each line, the image forming device shown in FIG. 25 is able to control the latent image formation on the photo conductor 1001 by controlling the time of light emission of the semiconductor laser in the semiconductor laser unit 1009.
In a scanning optical system having the above configuration, a turnable deflector such as the polygon scanner is used. However, the distance between one reflection plane and the rotational axis of the deflector may change, as a result, there arises non-uniformity of the scanning speed of the laser beam (that is, the speed of the light spot moving on the medium during scanning), and causes image degradation. To meet the requirements of high image quality, the scanning non-uniformity should be eliminated.
On the other hand, when a multi-beam optical system is used as a light source, if the wave-lengths of individual lasers are somehow different, the irradiation positions of the laser beams on the medium may differ from each other. This causes different scanning lengths of individual lasers and therefore image degradation. To attain high image quality in such a case, it is also required to make corrections to the scanning length.
In the prior art, for example, the Japanese Unexamined Patent Publications (Kokai) No. 11-167081 and No. 2001-228415 disclose methods for controlling the light spot positions along a scanning line by changing the frequency of the pixel clock.
There is also a well-known method as illustrated in FIG. 26, in which the number of clocks is counted in a period associated with the scanning from a photo detector A 1107 to a photo detector B 1108 situated at the two ends of the photo conductor 1105, respectively, to detect the scanning speed, and thereby to control the rotational speed of the polygon mirror 1104.
In the above related art involving changing the frequency of the pixel clock (it is the so-called “Frequency Modulation”), however, the configuration of the control unit for the pixel clock becomes complicated, and becomes more complicated if the frequency modulation width becomes smaller. Therefore, this technique is not suitable to delicate control.
The above non-uniformity in scanning can be ascribed to the rotational jitter of the deflector, and expansion and contraction of the scanning lens 1002 when the temperature changes. Thus, it is difficult to eliminate the scanning non-uniformity even when the same reflection plane of the deflector is used to reflect the laser beam, because there is a limitation to the precision of controlling the rotational motor of the deflector.